why does preamp improve sound quality?

A good pre-amp doesn't improve the sound quality, it just does not degrade it as much as a lesser quality pre-amp.

The source components are designed to feed an electrical signal to the designed for source inputs of s source component.
The outputs of a preamp are electrically set up to go into an amp.
Once in awhile some source components can be directly plugged into some amps and it works beautifully.
More often the direct source-to-amp DOES NOT WORK*.
(I mean it does not sound very good)
It does not work well because of impedance problems, and source/amp design parameters.
So if your source sounds better with the preamp in place, clearly the souce and amp alone are NOT a good combination for that sort of connection.
They would probably sound even better with a great preamp. They just are not synergistic aa a direct coupling.

I have tried going direct several times but having the preamp in the chain has always been better. Arthur

A good preamp can improve the impedance matching between sourced and amp, and also give the music more body and life, primarily from extra gain. I've tried it both ways and have preferred a good preamp every time.